Control devices which respond to ambient light conditions have been extensively employed for turning on and off street and highway lamps as well as lighting for other public facilities. These devices generally utilize a light sensitive element which has a low electrical resistance when illuminated and a relatively high resistance when it is in the dark.
In a typical prior device, a photocell is connected in series with a lighting load, a power source and either an electromagnetically actuated relay of the type having a hinged armature and normally closed contacts or a thermally actuated relay having normally closed contacts. When the cell is in the dark, its resistance is high and the current flowing through the relay coil is too low to open the relay contacts. Since the contacts in series with the load are closed when the relay is deenergized, current flows through the lighting load. When the light impinges on the cell, its resistance decreases causing current to flow through the relay coil thereby opening the relay contacts and disconnecting the load. These devices are constructed so that they are fail safe; that is, the lighting load is energized in the event the photocell or relay should fail. They may be used to control one or a plurality of street lamps and generally consume little power. Examples of such control switches are disclosed in my U.S. Pat. Nos. 3,250,951 granted May 10, 1966 and 3,093,744 granted June 11, 1963.
A disadvantage encountered with these prior art devices is that they are extremely difficult to adjust so that the lighting load will be turned on and off at a precise predetermined level of ambient illumination. This is primarily because the spring tension on the relay contacts is variable and difficult to set. In a typical control switch, the relay will drop out for values of illumination which are much less than those required to pick up the relay, the ratio of drop-out to pick-up sometimes being as high as 4.0. Consequently, the street lamp may be turned on and off at levels of ambient illumination other than the desired one and, in lighting systems in which each lamp is controlled by an individual switch, the lights may turn on and off in an erratic pattern.
Until recently, the inability to accurately control the operation of such switches has not been an economically important factor because of the relatively low cost of electrical energy. However, as the cost of generating and transmitting electrical energy has increased, it has become of greater importance that lamps used for public lighting be turned off as soon as the ambient light has reached a predetermined value at which they are not longer needed, and that the lamps not be turned on again until the ambient level has fallen to the same value. As an example, it would be desirable to have a control switch which would both open and close at an ambient illumination of approximately one footcandle with the ratio of drop-out to pick-up being as close to unity as possible.
Accordingly, it is an object of my invention to provide a light responsive control device which can energize or deenergize a load when the incident illumination is within a narrow predetermined range.
Another object is to provide a control device wherein the pick-up and drop-out characteristics are substantially identical from unit to unit and which does not require adjustment in the field.
Other objects are to provide a control device which is fail safe in operation, has a simple and compact construction, is easy to install, will operate for many years and is relatively inexpensive.